[volt-nuts] New member with 3457A problem

[Jerry Hancock]

Hello again, Just for the record, after spending another dozen or so hours on this problem, I found U216, “Precharge offset adj” to have a low resistance from pins 1,5,6 to pin 4, essentially one of the 4 transistors in the CA3096 is shorted (11.28 ohms) though not completely.  I found this by scoping the signal coming into the input amplifier at pin 10 of U101, “Input Hybrid”, while putting the meter into an auto cal loop.  This problem was difficult to track down as unlike the other Service Manuals, there are no waveforms available in the 3457a manual.  I noticed that during the precharge cycle the waveform was taking a large negative dive instead of two relatively equal pulses, one that reads the signal and the other during precharge, I assume.  This negative voltage from the precharge offset was then not being drained efficiently when the leads were open resulting in a negative to zero open voltage displayed on the meter compared to the typical ~+2.5V.

So for the record, if you get a “0 Hardware error” followed by a 512 or 560 (512 + 32 + 16 errors signaled) in reply to the “auxerr?” command, take a look in the area of U216 and check the four transistors that are in the CA3096E.

Lastly, as I commented on another site, after reading through the 3457a theory of operations, that there are a number factors that need to be calibrated out of a meter to ensure accuracy and repeatable measurements.  In this case, obviously the input amplifier needs to have a stable offset voltage (zero).  The precharge offset charges the approximate 10pf of stray capacitance out of the input amplifier to keep it from impacting the readings.  In conclusion, it takes more than an accurate ADC, gain resistors, etc to make a good meter.  I followed a number of threads where people were attempting to make their own 5.5 digit meter, I played around with some circuits myself, but nowhere did I see stray capacitance taken into account.

Thank you for reading.  By the way, there is what looks like a nice 3458a on craigslist in the SF Bay area.  Initial price of $2,900 was dropped last night to $1,950.  I wish I had some leftover cash from Christmas!

Jerry

On Jan 3, 2016, at 6:26 PM, Jerry Hancock <jerry at hanler.com> wrote:

> I’ve been reading the list for a while, never had a reason to post until now.
> 
> I have an HP 3457a, actually two of them, both work great, accurate, stable, etc., except one won’t autocal without an error 512 in auxerr - ohms recharge offset.  I’ve had the meters side by side for hours the past few days and can’t seem to locate the problem.  There is a difference in the Ohms precharge offset in the failing meter.  It looks like a factor mod was performed to lower U216 below +15 by adding resistors to legs 13 and 10.  I know there were service notes sent out by HP in 1992 related to the 512 error. the other thing of note is that occasionally when I power on the meter, it will also get a 16 error, which is the offset.  I ran diagnostic 4 and calibrated the offset but occasionally the meter will not pass the test no mater how the offset it adjusted.  For those of you not familiar, the diagnostic 4 puts the meter in a loop displaying HI or LO depending on the adjustment until the repair tech (me in this case) adjusts the offset correctly.  At that point the meter displays ‘Passed’.  When the meter fails, usually when cold, the display oscillates between HI/LO like the ADC doesn’t have a connection and the input is floating.  I tried swapping U101, the input hybrid, between the two units without luck. 
> 
> So I am wondering if anyone on the list has run into this problem in the past, and if so, the solution would be appreciated.  
> 
> Thanks.
> 
> By the way, I have a number of voltmeters, can’t seem to get enough, and am pretty good with 3455A repairs having four of them.  My black face 3455A is one of my favorites.  I also have 2 x 3456A, 2 x 3457A, 3 x 3478a, 2 x 3476, a bunch of Keithely (191, 178, 199, 195, 196, etc) and others, mostly older HP units like the 400.  I am currently eyeing a 3458a.  I guess I am a volt-nut at heart.  
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